Photoconductive elements useful, for example, in electrophotographic copiers and printers are composed of a conducting support having a photoconductive layer that is insulating in the dark but becomes conductive upon exposure to actinic radiation. To form images, the surface of the element is electrostatically and uniformly charged in the dark and then exposed to a pattern of actinic radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface and dissipate the surface charge. This leaves in non-irradiated areas a charge pattern known as a latent electrostatic image. The latent image can be developed, either on the surface on which it is formed or on another surface to which it is transferred, by application of a liquid or dry developer containing finely divided charged toner particles.
Photoconductive elements can comprise single or multiple active layers. Those with multiple active layers (also called multi-active elements) have at least one charge-generation layer and at least one n-type or p-type charge-transport layer. Under actinic radiation, the charge generation layer generates mobile charge carriers and the charge transport layer facilitates migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge and form the latent electrostatic image.
Also useful in photoconductive elements are charge barrier layers, which are formed between the conductive layer and the charge generation layer to restrict undesired injection of charge carriers from the conductive layer. Various polymers are known for use in barrier layers of photoconductive elements. For example, U.S. Pat. No. 5,128,226 discloses a photoconductor element having an n-type charge transport layer and a barrier layer, the latter comprising a particular vinyl copolymer. U.S. Pat. Nos. 4,082,551 and 3,428,451 disclose a two-layer system that includes cellulose nitrate as an electrical barrier. U.S. Pat. No. 5,681,677 discloses photoconductive elements having a barrier layer comprising certain polyester ionomers. U.S. Pat. No. 4,971,873 discloses solvent-soluble polyimides as polymeric binders for photoconductor element layers, including charge transport layers and barrier layers.
Still further, a number of known barrier layer materials function satisfactorily only when coated in thin layers. As a consequence, irregularities in the coating surface, such as bumps or skips, can alter the electric field across the surface. This in turn can cause irregularities in the quality of images produced with the photoconductive element. One such image defect is caused by dielectric breakdowns due to film surface irregularities and/or non-uniform thickness. This defect is observed as toner density in areas where development should not occur, also known as breakdown.
The known barrier layer materials have certain drawbacks, especially when used with negatively charged elements having p-type charge transport layers. Such elements are referred to as p-type photoconductors. Thus, a negative surface charge on the photoconductive element requires the barrier material to provide a high-energy barrier to the injection of positive charges (also known as holes) and to transport electrons under an applied electric field. Many known barrier layer materials are not sufficiently resistant to the injection of positive charges from the conductive support of the photoconductive element. Also, for many known barrier materials the mechanism of charge transport is ionic. This property allows for a relatively thick barrier layer of previously known barrier materials, and provides acceptable electrical properties at moderate to high relative humidity (RH) levels. Ambient humidity affects the water content of the barrier material and, hence, its ionic charge transport mechanism. Thus, at low RH levels the ability to transport charge in such materials decreases and negatively impacts film electrical properties. A need exists for charge barrier materials that transport charge by electronic as well as ionic mechanisms so that films are not substantially affected by humidity changes.
Condensation polymers of polyester-co-imides, polyesterionmer-co-imides, and polyamide-co-imides are all addressed in:                1. Sorriero et al. in U.S. Pat. No. 6,294,301.        2. Sorriero et al. in U.S. Pat. No. 6,451,956.        3. Sorriero et al. in U.S. Pat. No. 6,593,046.        4. Sorriero et al. in U.S. Pat. No. 6,866,977.        5. Molaire et al. in U.S. patent application Ser. No. 10/888,172.        
These polymers have as a repeating unit a planar, electron-deficient, tetracarbonylbisimide group that is in the polymer backbone. The polymers are either soluble in chlorinated solvents and chlorinated solvent-alcohol combinations, or they contain salts to achieve solubility in polar solvents. In all cases, care must be taken not to disrupt the layer with subsequent layers that are coated from solvents, as this may result in swelling of the electron transport layer, mixing with the layer, or dissolution of part or all of the polymer. Furthermore, salts can make the layer subject to unwanted ionic transport. Thus there is a need for polymers with planar, electron-deficient tetracarbonylbisimide groups and do not contain salts that can be coated from solvents, but will not be soluble or miscible with subsequent solvents or layers. Further, there is a need for polymers with planar, electron-deficient tetracarbonylbisimide groups and do not contain salts that can be coated from non-chlorinated solvents. Further, there is a need for polymers with planar, electron-deficient tetracarbonylbisimide groups and do not contain salts that can be coated from solvents, but will not be soluble or miscible with subsequent solvents or layers.
Another disadvantage to the condensation polymers of polyester-co-imides, polyesterionomer-co-imides, and polyamide-co-imides addressed above is they generally consist of monomers other than the planar, electron-deficient tetracarbonylbisimide groups. The level of electron transport agent in the condensation polymers is generally limited because common condensation monomers are also incorporated into the polymer to achieve good mechanical as well as good solubility properties. For example, although the alcohol portion of the polyester may consist of a planar, electron-deficient tetracarbonylbisimide group, the acid portion is generally an aliphatic or aromatic diacid that does not transport charge. It is generally necessary to have the comonomer as a spacer in order to achieve good solubility, even when chlorinated solvents are used. In fact it is difficult to prepare a soluble condensation polymer where all of the diol groups consist of the planar, electron-deficient tetracarbonylbisimide groups. Generally other diols and diacid monomers are used to prepare the polyesters described above. This limits the amount of planar, electron-deficient tetracarbonylbisimide group that can be incorporated into the polymer, and thus limits the amount of charge that can be transported through these layers. The same limitation is true for the polyamides described in the patents above, where the planar, electron-deficient tetracarbonylbisimide group is generally only a fraction of the acid portion used in the polymer, and a common amine that does not transport electronic charge is used as the diamine monomer portion of the polyamide.
Japanese Kokai Tokkyo Koho 2003330209A to Canon includes polymerizable naphthalene bisimides among a number of polymerizable electron transport molecules. Some of the naphthalene bisimides contain acrylate functional groups. The monomers are polymerized after they are coated onto an electrically conductive substrate. However this approach does not ensure the full incorporation of all of the monomers. Some of the functional groups would not react to form a film and could thus be extracted during the deposition of subsequent layers. This would result in a layer that was not the same composition as deposited before polymerization. Further, it would allow for the unwanted incorporation of the electron transport agent into the upper layers of the photoreceptor by contamination of the coating solutions. Thus, the need remains for a well characterized electron transport polymer that can be coated and crosslinked completely to produce a layer that will transport electrons between layers of a photoreceptor without contaminating subsequent layers.
Japanese Kokai Tokkyo Koho 2003327587A to Canon describes the synthesis of naphthalene bisimide acrylate polymers. The polymers were coated from solution onto “aluminum Mylar” and crosslinked to harden the layer to form crack free films. Mobility measurements were made. No layer was coated upon this layer and no crosslinking chemistry for the polymer is described. A photoreceptor is not described. The need exists to form an insoluble film from a polymer that can transport electrons and has active sites for crosslinking that result in a film that can be overcoated with subsequent layers to form a photoreceptor. The crosslinking should be done either thermally or with UV light. The naphthalene bisimide polymer must be completely soluble in the coating solution and crosslink so the layer is intact when subsequent layers are coated upon the naphthalene bisimide layer.
Photoconductive elements typically are multi-layered structures wherein each layer, when it is coated or otherwise formed on a substrate, needs to have structural integrity and desirably a capacity to resist attack when a subsequent layer is coated on top of it or otherwise formed thereon. Such layers are typically solvent coated using a solution with a desired coating material dissolved or dispersed therein. This method requires that each layer of the element, as such layer is formed, should be capable of resisting attack by the coating solvent employed in the next coating step. A need exists for a negatively chargeable photoconductive element having a p-type photoconductor, and including an electrical barrier layer that can be coated from an aqueous or organic medium, that has good resistance to the injection of positive charges, can be sufficiently thick and uniform that minor surface irregularities do not substantially alter the field strength, and resists hole transport over a wide humidity range. Still further, a need exists for photoconductive elements wherein the barrier layer is substantially impervious to, or insoluble in, solvents used for coating other layers, e.g., charge generation layers, over the barrier layer. It would also be an advantage to have polymers that form barriers that can be coated out of non-chlorinated solvents. Solvents such as toluene and alcohols are more desirable environmentally because the vapors are not as noxious as those of chlorinated solvents, and the disposal of the excess coating solutions is not as dangerous if chlorinated solvents are not used. Thus, it is a goal to have a barrier layer that can be manufactured and coated from “green” solvents.
Accordingly, a need exists for a negatively chargeable photoconductive element having a p-type photoconductor, and including an electrical barrier layer that can be coated from an aqueous or organic medium, that has good resistance to the injection of positive charges, can be sufficiently thick and uniform that minor surface irregularities do not substantially alter the field strength, and resists hole transport over a wide humidity range. Still further, a need exists for photoconductive elements wherein the barrier layer that is itself coated from non-chlorinated solvents and is substantially impervious to, or insoluble in, solvents used for coating other layers, e.g., charge generation layers, over the barrier layer.
Photoconductive elements comprising a photoconductive layer formed on a conductive support such as a film, belt or drum, with or without other layers such as a barrier layer, are also referred to herein, for brevity, as photoconductors.